Last Modified: Jul 18, 2026
Affected Product(s):
BIG-IP All
Known Affected Versions:
11.5.4, 11.5.4 hf1, 11.5.4 hf2, 11.5.4 hf3, 11.5.4 hf4, 11.6.0, 11.6.0 hf1, 11.6.0 hf2, 11.6.0 hf3, 11.6.0 hf4, 11.6.0 hf5, 11.6.0 hf6, 11.6.0 hf7, 11.6.0 hf8, 11.6.1, 11.6.1 hf1, 11.6.1 hf2, 11.6.2, 11.6.2 hf1
Fixed In:
12.0.0, 11.6.3, 11.5.5
Opened: Apr 02, 2015 Severity: 2-Critical Related Article:
K47804233
When the server TCP connection is under timewait closing state, if a new client connection is initiated toward the server under the BIG-IP SYN-Cookie mode, the server respond with ACK instead of SYN+ACK for the SYN received. The BIG-IP system drops this ACK and retransmit the SYN, till timeout occurs.
Longer establishment time and retry.
-- FastL4 is under SYN-Cookie mode. -- The previous server connection is under time wait close state. -- New client connection is reusing the port to get to the same server TCP connection.
None.
When ACK is received instead of SYN+ACK in this case, the system resets the server side to quick recovery, ensuring a quick recovery.